Drive is making a clunking noise

I'm having a somewhat of a strange problem. I have 2 WD external disks. One is 1TB and one is 500gb. The 1TB drive is working fine but Vista (64bit) decided not to recognize my 500gb drive anymore and is now listing it as unknown device.

Furthermore, drive is making a clucking noise if you can imagine. When it does that noise Vista shows it on my computer but same second throws it off the list. And that repeats itself until Explorer freezes. And I can see the drive for 1sec in the Computer Management -> Drives section and then it dissapears again.